Semiconductor lasers are used in optical transmitters for transmitting data over optical fibers in various applications. In cable TV (CATV) applications, for example, optical transmitters may be used to transmit video and broadband signals to customers using dense wavelength division multiplexing (DWDM). The Data Over Cable Service Interface Specification (DOCSIS) standard governs the transmission of high-bandwidth data to an existing CATV system to provide internet access over an existing CATV infrastructure. The most recent DOCSIS 3.1 standard allows higher RF frequencies (e.g., 1.2 GHz and above), which increases bandwidth.
When modulating semiconductor lasers in optical transmitters, higher RF frequencies may present problems and challenges. One problem is caused by chirp in the output of the semiconductor laser, which is a change in frequency as a function of time. In particular, chirp may lead to signal degradation of a modulated optical signal, especially for higher RF frequencies over longer transmission distances. To meet the requirements of the most recent DOCSIS 3.1 specification, the semiconductor lasers used in optical transmitters should be capable of supporting higher RF frequencies (e.g., 1.2 GHz and above) with lower chirp (e.g., less than 60 MHz/mA).
Chirp may be reduced by using external cavity lasers with an external grating as an exit reflector; however, external cavity lasers present problems with optical coupling and with the packaging. External modulators may also be used to reduce chirp but may not be capable of providing enough power for use in an optical transmitter for CATV applications.